When a device formation layer made of a different semiconductor material from that of a substrate is to be formed on the substrate, damage such as cracks disadvantageously occurs due to a difference in lattice constant between the substrate and the device formation layer.
Considering this problem, Patent Literature 1 discloses a technique for suppressing damage such as cracks. According to this conventional technique, a buffer layer is provided between a substrate and a device formation layer. The buffer layer includes a first layer made of AlN, a fourth layer made of AlMGaN, a second layer made of GaN, and a third layer made of AlGGaN, which layers are repeatedly stacked in this order. Symbol M indicates an element selected from between In (indium) and B (boron).
According to the technique of the Patent Literature 1, Al content rates of the third and fourth layers are set lower than that of the first layer. The Al content rates of the third and fourth layers are gradually changed in a thickness direction. Due to these factors, this conventional technique can relax the stress due to the difference in the lattice constant between the substrate and the device formation layer and can improve crystallinity to some extent.